Characteristics of Sediment-related Disasters in July 2012 in the Aso Region of Kyushu Island, Japan
Nobuaki KATO
1*
, Masayuki MIYASE
1
, Masaharu NAKAYAMA
2
, Kosei MOTODA
2
and Toshihiro NAKAMURA
2
1 SABO & Landslide Technical Center
2 Sediment Control Division, Kumamoto Prefectural Government
*Corresponding author. E-mail: kato@stc.or.jp
INTRODUCTION
Record heavy rainfall occurred from July 11 to 14, 2012, in northern Kyushu, Japan. In the Aso region, Aso City, Takamori Town, and Minami-Aso Village, the rainfall caused 85 sediment-related disasters and left 25 people dead or missing. This paper reports the characteristics of the sediment-related disasters.
OVERVIEW OF THE REGION AND SEDIMENT MOVEMENTS
The Aso region is a caldera zone located in Kumamoto Prefecture, Kyushu Island, Japan.
The Aso caldera is one of the largest calderas in Japan. The somma is 25 km north–south and 18 km east–west. The basin of the caldera is urbanized and cultivated, and about 46,000 people live in and around it.
The area has a high potential for sediment-related disasters, with 221 debris-flow torrent channels distributed around the central cones and internal slopes of the somma. Past sediment flows have caused much damage, including in 1990 when a frontal rain event produced 74 debris-flow torrents that killed 13 people.
A hyetograph for July 11 to 14, 2012, at the Aso-Otohime observing station (AMeDAS) is shown in Fig. 1. The maximum 1-hour rainfall and 24-hour rainfall values during this event were the largest ever recorded at the station. From 3 to 6 am on July 12, over 80 mm/h of rain fell, producing most of the debris flows that occurred during this event. The rainfall was concentrated in the north, especially the northwestern part of the caldera. At the Kano observing station, which is located in the northwestern part of the somma, the total rainfall was 890 mm. The Takamori observing station located to the south recorded 293.5 mm during the same period.
Fig.2 shows the elevation, debris-flow torrents, and sediment movements that were detected by aerial photographs. In total, 4,356 areas of sediment movement were detected.
Among these, 643 failures were detected outside the debris-flow torrent channels and
dangerous slopes. The sediment movement was concentrated around the central cones and the northeast part of the inner slopes of the somma. Thus, the concentrated areas of rainfall (northwest) and sediment movements (center and northeast) were incongruent. There were no distinct differences in topographical and geological conditions between the northeast and northwest of the caldera, and the reasons for the concentration of sediment movement in the northeast have not been identified.
OVERVIEW OF THE TOI RIVER BASIN
The committee on countermeasures for sediment-related disasters in the Aso region (2013) has categorized sediment movements into the following five types: 1) collapses and debris flows at the caldera wall, 2) collapses in talus deposits, 3) collapses in pyroclastic deposits on hillslopes, 4) collapses on steep slopes of the central cones, and 5) collapses and debris flows on the steep slopes around Mt. Neko.
The Toi River, which has a catchment area of 0.35 km2, was designated as a debris-flow torrent channel. The debris flows in the Toi River were categorized as Type 1 and had the following general features (see Fig. 3).
・ Surface failures occurred in upstream pastures and downstream forestland. Sediments generated upstream stopped in the river channel. Sediments generated downstream travelled down the stream as debris flow, killing one person.
・ The Toi River did not experience a sediment disaster in 1990, when a debris-flow torrent occurred in the nearby the Nakazono-3 River. In contrast, in 2012, only a few collapses were detected in the Nakazono-3 River basin.
・ The alluvial fan of the Toi River has been designated as a sediment-related disaster warning area. Because of the large amount of water supplied from the Toi and the Nakazono-3 Rivers, flood damage reached this warning area in 2012.
CONCLUSION
The features of the sediment-related disasters in the Aso region in 2012 are summarized as follows:
・ 4,356 failures occurred due to record heavy rainfall.
・ In the caldera wall, debris flow occurred in small drainage basins and zero-order channels, as well as in debris-flow torrents.
・ Sediment-related disasters were categorized into five types.
・ In some river basins, flood damage was extensive and far-reaching.
・ In many cases, countermeasure works protected residents from debris flow.
Because it is difficult to implement structural countermeasures to protect against all debris- flow torrents, non-structural measures are also essential in the Aso region to reduce the risk of sediment-related disasters.
0 200 400 600 800 1000
0 20 40 60 80 100 120 140
6 12 18 24 6 12 18 24 6 12 18 24 6 12 18 24
July 11th July 12th July 13th July 14th
cumulative rainfall(mm)
hourly rainfall (mm/h)
hourly rainfall cumulative rainfall
Fig.1 Hyetograph for July 11 to 14, 2012, at the Aso-Otohime observing station (AMeDAS)
Keywords: Aso region, sediment-related disasters, caldera, debris-flow torrent Fig.2 Map of sediment movements in
the Aso region
Fig.3 Schematic of the Toi River basin
